The present disclosure relates generally to Universal Serial Bus (USB) devices. More particularly, the present disclosure relates to power savings for USB devices.
FIG. 1 shows a prior art full-speed USB communication system 100 in which a USB host or hub 102 is connected to a full-speed USB device 104. Referring to FIG. 1, USB host or hub 102 includes a USB host controller transceiver 106 having a differential output. To detect the presence of USB devices 104, each terminal of the differential output has a respective pull-down resistor R1, R2 each having a resistance of 15 kΩ±5%.
Full-speed USB device 104 includes a full-speed USB device transceiver 108 having a differential output. A pull-up resistor R3 having a resistance of 1.51 kΩ±5% is connected to the D+terminal to indicate that USB device 104 is a full-speed USB device.
FIG. 2 shows a prior art low-speed USB communication system 200 in which USB host or hub 102 of FIG. 1 is connected to a low-speed USB device 204. Referring to FIG. 2, low-speed USB device 204 includes a low-speed USB device transceiver 208 having a differential output. A pull-up resistor R4 having a resistance of 1.5 kΩ±5% is connected to the D-terminal to indicate that USB device 204 is a low-speed USB device.
With a pull-up voltage V+=3.3V, these arrangements generate a minimum current I=200 μA, as shown on the D+ line in FIG. 1, and on the D− line in FIG. 2. While 200 μA is not a significant current for a large device such as a computer, it constitutes a significant power drain for smaller portable devices such as personal digital assistants, cell phones, and the like.